Medical radionuclide imaging (Nuclear Medicine) is a key component of modern medical practice. This methodology involves the administration, typically by injection, of tracer amounts of a radioactive substance, which subsequently localizes in the body in a manner dependent on the physiologic function of the organ system being studied. The radiotracer emissions, most commonly gamma photons, are imaged with a detector outside the body, creating a map of the radiotracer distribution within the body. When interpreted by an appropriately trained physician, these images provide information of great value in the clinical diagnosis and treatment of disease. Typical applications of this technology include detection of coronary artery disease (thallium scanning) and detection of cancerous involvement of bones (bone scanning). The overwhelming bulk of clinical radionuclide imaging is performed using gamma emitting radiotracers and detectors known as “gamma cameras”.
Gamma cameras typically consist of a large scintillation crystal (e.g. sodium iodide) having the property of emitting light when struck by gamma photons. Affixed to the rear of this crystal are multiple photomultiplier tubes with associated circuitry to detect the light flashes and to locate their position within the scintillation crystal. In front of the crystal is a collimator, typically consisting of several millimeters of lead with multiple holes penetrating it. The collimator serves to absorb all incoming photons except those approaching the crystal generally from the appropriate direction. The crystal, photomultiplier tubes and associated circuitry are typically enclosed in a large lead case that serves to shield the detector from unwanted external radiation. The entire apparatus is mounted on a gantry with a motorized apparatus for positioning the detector near the patient.
A gamma camera provides a two-dimensional image of radiotracer distribution. However, the distribution of radiotracers within the body is typically three-dimensional. The technique of single photon emission tomography (SPECT) is used to create three-dimensional, tomographic images similar to a “radionuclide CT scan” by using computer processing to “reconstruct” the three-dimensional tracer distribution from a series of two-dimensional, gamma camera images obtained from multiple angles around the patient. This is almost universally accomplished by mounting one or more gamma cameras to a motorized gantry and orbiting them around the patient. The data thus acquired is then processed to yield the three-dimensional images.
The three-dimensional SPECT images have been demonstrated to provide higher image contrast and to reduce apparent overlap of body structures. SPECT imaging is now considered to be the state-of-the-art in radionuclide imaging of the heart and now accounts for more than half of all cardiac nuclear imaging performed in the United States.
Despite its many advantages, SPECT imaging is not yet available to all patients who might benefit from it. Current SPECT instrumentation has a number of disadvantages which have impeded its wider implementation.
Current SPECT systems are bulky, typically requiring a large, dedicated room to house them. The collimating systems are relatively inefficient, blocking a high percentage of emitted radiation. Thus, most new clinical systems simultaneously utilize two or more gamma camera detectors mounted on a single gantry. Since each detector typically weighs several hundred pounds, the supporting gantry must be large and heavy. Most SPECT installations require specially constructed rooms with added floor reinforcement. Since accurate image reconstruction requires precise detector placement, SPECT systems require heavy positioning systems consisting of motors and gearing capable of moving and positioning hundreds of pounds of apparatus to a precision of approximately a millimeter. These systems are necessarily large, heavy and expensive.
Although there is great medical need to image patients in a variety of settings, including doctors' offices, emergency rooms and intensive care units, the great size and bulk of current SPECT systems has required them to be in a fixed location, typically a hospital Radiology or Nuclear Medicine department. There are significant medical and patient convenience advantages to having cardiac SPECT imaging performed in the immediate presence of the attending Cardiologist. Many studies have shown that the cost of care delivered in an outpatient office setting is less than that of a hospital setting. Despite these compelling factors, the size and cost constraints of current systems have greatly limited their penetration into the community and have particularly limited their availability in physicians' offices. In addition, the large space requirements of current systems have imposed significant costs on hospitals providing SPECT services.
Current SPECT systems have additional limitations. As the gamma cameras orbit around the patient, large multi-conductor cables are required to carry power and data to and from each detector. These cables are repeatedly flexed during system operation and are a frequent cause of equipment breakdown.
The large and heavy nature of existing systems has dictated a mechanical gantry design that is highly stable, yet cost effective. This has resulted in systems in which the patient must lie in a supine (flat on the back) position on a narrow platform that extends into a vertically oriented gantry. In order to permit the detectors to be as close as possible to the chest and to enable the large, moving detectors to safely pass around the patient, current systems require the patient to maintain one or both arms in an uncomfortable position held over the head. This is painful for most patients and impossible for some. In addition, the supine position is uncomfortable for many patients, particularly for those with back problems. Many patients feel claustrophobic when inside the equipment. The narrow platform required to permit camera rotation around the patient is uncomfortable for large individuals and is often perceived as insecure or precarious by those undergoing scans. Also, the fact that the patient is partially enclosed by the equipment during imaging may serve to limit physician or nursing access to critically ill patients.